Magnetic field sensors (e.g., rotation detectors) for detecting ferromagnetic articles and/or magnetic articles are known. The magnetic field associated with the ferromagnetic article or magnetic article is detected by a magnetic field sensing element, such as a Hall element or a magnetoresistance element, which provides a signal (i.e., a magnetic field signal) proportional to a detected magnetic field. In some arrangements, the magnetic field signal is an electrical signal.
The magnetic field sensor processes the magnetic field signal to generate an output signal that changes state each time the magnetic field signal crosses a threshold, either near to peaks (positive and/or negative peaks) or near to some other level, for example, zero crossings of the magnetic field signal. Therefore, the output signal has an edge rate or period indicative of a speed of rotation of the ferromagnetic or magnetic object, for example, a gear or a ring magnet.
One application for a magnetic field sensor is to detect the approach and retreat of each tooth of a rotating ferromagnetic gear, either a hard magnetic gear or a soft ferromagnetic gear that is back-biased with a magnet. In some particular arrangements, a ring magnet having magnetic regions (permanent or hard magnetic material) with alternating polarity is coupled to the ferromagnetic gear or other rotating device such as a wheel axle or is used by itself and the magnetic field sensor is responsive to approach and retreat of the magnetic regions of the ring magnet. In other arrangements, a gear is disposed proximate to a stationary magnet and the magnetic field sensor is responsive to perturbations of a magnetic field as the gear rotates.
In one type of magnetic field sensor, sometimes referred to as a peak-to-peak percentage detector, one or more threshold levels are equal to respective percentages of the peak-to-peak magnetic field signal. One such peak-to-peak percentage detector is described in U.S. Pat. No. 5,917,320 entitled “Detection of Passing Magnetic Articles While Periodically Adapting Detection Threshold” and assigned to the assignee of the present invention.
Another type of magnetic field sensor, sometimes referred to as a slope-activated detector (or peak-referenced detector), is described in U.S. Pat. No. 6,091,239 entitled “Detection Of Passing Magnetic Articles With a Peak Referenced Threshold Detector,” also assigned to the assignee of the present invention. In the peak-referenced magnetic field sensor, the threshold signal differs from the positive and negative peaks (i.e., the peaks and valleys) of the magnetic field signal by a predetermined amount. Thus, in this type of magnetic field sensor, the output signal changes state when the magnetic field signal comes away from a peak or valley of the magnetic field signal by the predetermined amount.
It should be understood that, because the above-described peak-to-peak percentage threshold detector and the above-described peak-referenced detector both have circuitry that can identify the positive and negative peaks of a magnetic field signal, both such detectors include a circuit portion referred to herein as a “peak identifier”, which is configured to detect positive peaks and/or negative peaks of the magnetic field signal. The peak-to-peak percentage threshold detector and the peak-referenced detector, however, each use the detected peaks in different ways to provide a so-called “threshold generator,” which is configured to use the identified peaks to generate one or more threshold levels against which the magnetic field signal can be compared. This comparison can result in a so-called “PosComp” motion signal that has an edge rate representative of a speed of movement, e.g., rotation, of the moving object.
In order to accurately detect the positive and negative peaks of a magnetic field signal, in some embodiments, the rotation detector can be capable of tracking at least part of the magnetic field signal. To this end, typically, one or more digital-to-analog converters (DACs) can be used to generate a tracking signal, which tracks the magnetic field signal. For example, in the above-referenced U.S. Pat. Nos. 5,917,320 and 6,091,239, two DACs are used, one (PDAC) to detect the positive peaks of the magnetic field signal and the other (NDAC) to detect the negative peaks of the magnetic field signal.
Some types of rotation detectors perform one or more types of initialization or calibration, for example, at a time near to start up or power up of the rotation detector, or otherwise, from time to time as desired. During one type of calibration, the above-described threshold level is determined.
Once the above-described threshold level is initially determined, various schemes may be used for updating the threshold level to ensure that the threshold level remains at the desired relationship with respect to the peak-to-peak magnetic field signal level. For example, as described in U.S. Pat. No. 6,525,531 entitled “Detection of Passing Magnetic Articles while Adapting the Detection Threshold” and assigned to the assignee of the subject invention, the positive and negative detected peak signals (PDAC and NDAC, respectively) freely track “outwardly” to follow the magnetic field signal as it increases above PDAC and decreases below NDAC, respectively, following which such detected peak signals are selectively allowed to move “inward” (i.e., PDAC decreases and NDAC increases) to the level of the magnetic field signal upon transitions of the PosComp signal. Such threshold signal updating may be performed following an initial calibration mode, such as during a “running mode” of operation.
Some moving objects, for example, rotating moving objects, which are sensed by the above-described magnetic field sensors, exhibit irregular motions or have irregular features. For example, a gear may have wobble as it rotates, it may have run out (asymmetry about its axis of rotation), or it may have irregularities in its mechanical dimensions, for example, some gear teeth may be wider or taller than others. Additionally, anomalies in the conditions associated with the sensor or detected moving objects can cause intermittent oscillations of the object or other changes in the magnetic field detection. For example, when the magnetic field sensor is used to detect wheel speed in an automobile Anti-Lock Brake System (ABS), potholes can result in temporary changes to the axis of rotation of a wheel and thus, in the air gap (i.e., the distance from the object to the magnetic field sensing element). Such irregularities can cause variations in the magnetic field signal that can lead to generation of thresholds that are not ideal, thereby resulting in a PosComp signal that has edges that are not accurately placed relative to cycles of the magnetic field signal associated with features of the moving object.
It would, therefore, be desirable to provide a magnetic field sensor that can accurately identify a threshold level associated with a magnetic field signal, accurate even in the presence of irregularities in the motion of, or in the mechanical characteristics of, the moving object being sensed and/or in the presence of intermittent conditions associated with the sensor system.